Identifying faults during the design phase is most cost-effective because defects found early in the software development lifecycle are cheaper to fix. During the design phase, faults can be detected through reviews and inspections before any code is written, thus avoiding the costs associated with later stages of development. According to the ISTQB CTFL Syllabus, early testing and defect detection are key principles, which emphasize that the cost of fixing defects increases the later they are found in the lifecycle​​.